In general, (meth)acrylic acid is produced by the following steps of:                obtaining a gas containing (meth)acrylic acid by gas-phase oxidation reaction;        supplying the gas to a condensing tower or a collecting tower to obtain a crude (meth)acrylic acid solution; and        purifying (meth)acrylic acid from the crude solution.        
As a purification process for (meth)acrylic acid, crystallization is used as well as distillation, diffusion, extraction and the like.
Crystallization may be carried out by merely cooling a crude (meth)acrylic acid solution; however, in industrial mass production, dynamic crystallization is mainly employed. In dynamic crystallization, a crude (meth)acrylic acid solution to be cooled is flowed downward in a coating film-like state along a heat transfer surface of which the reverse side is brought into contact with a cooling medium.
There are various crystallization devices for carrying out such dynamic crystallization. For example, some crystallization devices have crystallization tubes. In such a crystallization device, a heat medium flows outside of the crystallization tubes and a crude (meth)acrylic acid solution is circulated to be repeatedly supplied to the crystallization tubes. Such a crystallization device is generally equipped with a large number of crystallization tubes. The crystallization tube has a thin diameter, and therefore the surface area of the heat transfer surface is enlarged. As a result, the heat energy of a heat medium can be effectively transferred to a crude (meth)acrylic acid solution.
However, it is not easy to evenly grow a crystal on the heat transfer surface of thin and long crystallization tube. In particular, a crystal is unevenly deposited in the lower part of a crystallization tube and finally, may completely clog the crystallization tube in some cases. When a crystallization tube is completely clogged, a crude (meth)acrylic acid solution remains in the upper part above the clogged point. A (meth)acrylic acid crystal is obtained as a melted liquid through sweating step and melting step. In the steps, the remaining crude (meth)acrylic acid solution is mixed to cause a problem that the purity of (meth)acrylic acid is lowered. Further, when pressure significantly increases in the system due to clogging, (meth)acrylic acid may possibly leak out of a crystallization device.
Patent Document 1 discloses a technology for producing (meth)acrylic acid. The technology contains the steps of recovering the heat of a gas obtained by gas-phase oxidation reaction using a heat exchanger, and supplying the gas to a collecting tower in order to obtain a crude (meth)acrylic acid solution. In the technology, when the pressure in a reactor is increased due to clogging of the heat exchanger, the gas is supplied to the collecting tower through a bypass without passing the gas through the heat exchanger. However, the problem of a pressure increase in a reactor and the problem of clogging in a crystallization device are completely different from each other, and there still remains a problem that clogging in a heat exchanger cannot be suppressed by the technology.